1. Technical Field
The present invention relates to a ball screw device for converting rotational motion into linear motion.
2. Related Art
In industrial machines such as a machine tool and an injection molding machine, a ball screw device of a tube circulation type as shown in FIG. 8 is used as a device for converting rotational motion into linear motion. The ball screw device comprises: a screw shaft 1; and a nut 2 having an inner periphery provided with a ball screw groove 4 opposing a ball screw groove 3 formed in the outer periphery of the screw shaft 1. Then, a large number of balls 6 serving as rolling elements are arranged in a ball load rolling path 5 formed between the ball screw groove 3 and the ball screw groove 4. These balls 6 roll on between the ball screw grooves 3 and 4 in association with the revolution of the screw shaft 1 or the nut 2. A flat part 2a formed in the outer periphery of the nut 2 is provided with a ball circulating tube 7 for circulating the balls 6. The ball circulating tube 7 is fabricated by bending into a U-shape a tube material composed of metal. The flat part 2a of the nut 2 is provided with two circulating tube insertion holes 8 into each of which an end of the ball circulating tube 7 is inserted. The ball circulating tube 7 has a tongue section at each end, so that the balls 6 rolling on between the ball screw grooves 3 and 4 are scooped up from the ball screw grooves 3 and 4 by the tongue section of the ball circulating tube 7.
In this ball screw device, the balls compete with each other in the ball load rolling path 5 formed between the ball screw grooves 3 and 4 and in the ball return path formed in the ball circulating tube 7. This jamming of the balls can cause rapid wear-out to the balls 6, or alternatively damage to the surface of the balls 6. Thus, in order to avoid the jamming of the balls, a device has been proposed in which ball retaining pieces 9 fabricated of resin are respectively arranged between the balls as shown in FIG. 9 (see Japanese Published Unexamined Patent Application No. 2003-269563 which is hereinafter referred as Patent Document 1).
Nevertheless, in the ball screw device described in the Patent Document 1 given above, when the filling factor of the ball retaining pieces becomes small in the ball circulation path, the ball retaining piece 9 can be temporarily caught at a step 10 or a gap 11 appearing near the outlet of the ball circulating tube 7 or alternatively at the tongue section 7a of the ball circulating tube 7, as shown in FIGS. 10A-10C. This causes trouble. In order that such temporary catch of the ball retaining piece should be avoided, the filling factor of the ball retaining pieces 9 need to be managed strictly at the time of fabrication. Alternatively, accuracy needs to be improved in the machining of the circulating tube insertion hole 8. Nevertheless, these approaches that the filling factor of the ball retaining pieces 9 is managed strictly at the time of fabrication, and that the accuracy is improved in the machining of the circulating tube insertion hole 8 cause a problem of a cost increase. The invention has been devised by focusing attention on this point.